An ophthalmic lens, in particular, a contact lens, must be supplied with oxygen from air, the oxygen being needed for maintaining corneal health condition, and thus, high oxygen permeability is required. In recent years, a contact lens has been developed which uses siloxane monomers as raw materials to improve oxygen permeability.
High hydrophilicity, in addition to oxygen permeability, is required for a contact lens. A contact lens with high hydrophilicity is generally said to provide good wearing feel and to be wearable for an extended period of time with comfort. Hydrophilic monomers are generally used as raw materials in order to improve hydrophilicity of a contact lens.
To produce a contact lens having both high oxygen permeability and high hydrophilicity, both a siloxane monomer and a hydrophilic monomer are required as raw materials; however, since a siloxane monomer has high hydrophobicity in general and thus has poor miscibility with a hydrophilic monomer, a phase separation occurs, thereby making it difficult to produce a transparent contact lens. In order to solve this problem, hydrophilized siloxane monomers have been developed.
For example, Patent Documents 1 and 2 disclose hydrophilic contact lenses produced from polysiloxane containing hydrophilic side chains, wherein compounds containing polyether groups, hydroxyl groups, amide groups, and quaternary amine groups and the like are selected for the hydrophilic side chains; however, since the specifically disclosed polysiloxane contains comparatively many hydrophilic side chains per each molecule, each chain having a comparatively low molecular weight, the hydrophilicity and oxygen permeability are not adequately balanced, making it difficult to use said polysiloxane as a raw material for an ophthalmic lens with high oxygen permeability and high hydrophilicity.
Patent Document 3 discloses a molded plastic article formed from a polymerized component containing, as the hydrophilic side chains, a group having a polymerizable double bond, a hydroxyl group, an organosiloxane group, and an amide group. The polymerized component is said to make it possible to produce the molded plastic article with high transparency, high oxygen permeability, good water wettability, and good mechanical properties. However, the specifically disclosed compound has a short chain of siloxane, and thus, effective oxygen permeability cannot be obtained, and the compound has only one polymerizable functional group per each molecule, thereby making it difficult to obtain adequate strength.
Patent Documents 4, 5 and 6 disclose a hydrogel soft contact lens formed from a hydrophilic siloxane monomer and a copolymer thereof, in which the hydrophilic siloxane monomer contains a main chain obtained by linking polymerizable groups and polysiloxane chains by urethane or urea groups, where it contains as side chains fluorine-substituted hydrocarbon groups and polyoxyalkylene groups. Hydrophilic siloxane monomers containing urethane or urea groups have high intermolecular forces, making its copolymers also prone to have a comparatively high modulus of elasticity and leading to a loss in some cases of the flexibility which greatly influences the wearing feel of contact lenses.    [Patent Document 1] Japanese Patent Publication No. 62-29776    [Patent Document 2] Japanese Patent Publication No. 62-29777    [Patent Document 3] Japanese Patent No. 3,644,177    [Patent Document 4] International Publication WO 01/044861    [Patent Document 5] International Publication WO 2006/026474    [Patent Document 6] Japanese Patent No. 3,441,024